In the molding die industry, the material used for the die is required to have certain characteristics, namely, a high hardness to ensure sufficient strength and abrasion resistance needed to enable it to be used in products such as a die, and excellent machinability.
Conventionally, a prehardened steel is used as the die material. For the prehardened steel, the typical processes used to increase the strength (hardness) of ordinary steel such as annealing, machining, and quenching are not employed. Rather the steel is adjusted to a predetermined hardness and machined without the quenching treatment and then used for the die.
A particular prehardened steel, increased in hardness using precipitation hardening with the addition of Ni, Al, and Cu, is modified to have a bainite structure which is highly machinable, and as such, is useful as a steel which provides an improved hardness and comparatively good machinability. The steel is not required to be subjected to any quenching treatment after machining, and thus highly usable for die makers.
However, even if a highly machinable material is obtained, in order to machine a die having a complicated shape, an electric discharge machining is indispensable, as well as a laser machining, for example. Particularly, the electric discharge machining is used for an ultraprecise die specified for use in the fabrication of semiconductor components.
Also, the electric discharge machining enables the formation of a die regardless of the machinability, for example even for materials with poor machinability, and cemented carbide such as WC—Co, SK-3, and SKD61.
Although electric discharge machining is widely used for machining precise dies because it is usable for machining material having poor machinability and high hardness, it is actually uncommon to use the surface subjected to the electric discharge machining as it is without further surface treatment, and a polishing finish by manual polishing is typically subsequently performed to reduce the surface roughness, eliminate defects in a surface layer such as a microcrack, and improve the accuracy of the form. However, this process largely depends on the skill of a skilled worker and requires a long time, and therefore it would be desirable to improve the operational efficiency of the process.